This invention relates to combustion turbines powered by both gaseous and liquid hydrocarbon fuels. In particular, this invention relates to a method for preventing coking of liquid hydrocarbon fuel in a liquid fuel supply system of a gaseous fuel/liquid fuel combustion turbine.
Combustion turbines, such as those used for generating electric power, are often fueled by gaseous hydrocarbon fuel, but have access to an alternative liquid hydrocarbon fuel for use when the gaseous fuel is not available or is undesirable. While the combustion turbine operates with gaseous fuel, an adjacent liquid fuel supply system connected to a fuel distributor in the combustion turbine stores the liquid fuel in standby mode.
The liquid fuel supply system includes an arrangement of pipes and valves and is filled with liquid fuel for use when necessary or desired. Combustion of the gaseous fuel during operation of the combustion turbine produces high temperatures in the combustion chamber of the furnace and in the area adjacent the combustion chamber including the area occupied by the liquid fuel supply system. The liquid fuel supply system is filled with liquid hydrocarbon fuel, but also includes some oxygen and air trapped in the liquid fuel system. The combination of liquid hydrocarbon fuel, oxygen, and high temperatures in the liquid fuel supply system adjacent the combustion chamber of the turbine causes oxidation and partial decomposition of the liquid fuel in the liquid fuel supply system and produces coke in the liquid fuel supply system. This process is referred to as “coking” and the coke forms hard deposits in the liquid fuel supply system and clogs and fouls the associated valves and valve screens. Excessive coking and clogging interferes with effective liquid fuel transfer through the liquid fuel supply system and can require the combustion turbine to be shut down for cleaning of the liquid fuel supply system or replacement of its components.
This problem of coking has been addressed by frequently transferring liquid fuel from the liquid fuel supply system to exercise the system components and burn stagnant fuel. This, however, causes operational and financial problems by requiring weekly burning of stagnant liquid fuel, when operation of the combustion turbine with gaseous fuel is normally more economical and desirable.
Another proposed solution is to recirculate liquid hydrocarbon fuel in the liquid fuel supply system, rather than transfer the fuel entirely. This option, however, is complex and expensive and therefore undesirable.
Accordingly, there is a need for a simple and economically desirable method for alleviating coking in the liquid fuel supply system of a combustion turbine.